Recently, the liquid crystal display device has been in wide use as a monitor for a portable notebook personal computer or the like, or a display section for a liquid crystal television, a video integrated liquid crystal television or the like, and in various other fields. The liquid crystal display device basically comprises a backlight section and a liquid crystal display element section. As the backlight section, a type of an edge light system has frequently been used with a view to making the liquid crystal display device compact.
Conventionally, as a backlight, a type has been, in wide use in which at least one end face of a rectangular plate-like light guide is used as a light incident end face or light incoming end face, a linear or bar-shaped primary light source such as a straight tube fluorescent lamp is arranged along the light incident end face, a light emitted from the primary light source is introduced into the light guide through the light incident end face thereof, and the light is emitted from a light emitting face or light outgoing face which is one of two principal surfaces of the light guide. The light emitted from the light emitting face of the light guide predominantly has a direction of angle of 45° or less relative to the light emitting face. Therefore, a prism sheet is arranged on the light emitting face of the light guide so that the light is deflected to a required direction, e.g. a normal direction of the light emitting face.
Meanwhile, there has recently been made a demand for miniaturization and a reduction in power consumption regarding liquid crystal display devices of relatively small screen sizes such as portable electronic devices, e.g., cellular phones, portable game machines, etc., or indicators of various electric devices or electronic devices. Thus, an LED that is a point light source is used as the primary light source of the backlight in order to reduce power consumption. As the backlight that uses the LED as the primary light source, an example is described in JP(A)-7-270624 in which a plurality of LED's are one dimensionally arrayed along the light incident end face of the light guide in order to exhibit a function similar to that of the backlight which uses the linear primary light source. The use of the primary light source based on the one-dimensional arrangement of the plurality of LED's enables acquisition of a required light amount and uniformity of a luminance distribution on an entire screen.
In case of the backlight for use in the portable electronic devices, lightening and thinning of the devices have been strongly demanded. Therefore, the thickness of the light guide is generally set to a smaller value than that of the conventional backlight. In such a case, it may happen that a portion of the LED is positioned beyond the edge of the light incident end face of the light guide, that is, protrudes across a level of the light emitting face in the light emitting direction across a level of the light emitting face. The light emitted from the portion of the LED positioned beyond the edge of the light incident end face may directly enter the light deflecting prism sheet without passing through the light guide, i.e. without falling on the light incident end face. Such a dirty incident light is emitted from the prism sheet in a direction different from that of a normal case where the light is emitted from the light emitting face of the light guide and introduced into the prism sheet, to thereby cause a bright line observed in planar light emission, which has been desired to be eliminated.
In order to block the above-mentioned light which is introduced directly into the prism sheet from the LED, it may be designed that a light-blocking member is disposed on the light emitting face of the light guide in an end region on the side of the light incident end face, and the light deflecting prism sheet is disposed thereon. FIG. 12 is a schematic cross-sectional view of a planar light source device having such a configuration. In FIG. 12, an LED 102 is arranged so as to face a light incident face 141 of a light guide 104. The LED 102 has a lighting portion 121 and a non-lighting portion 122. A light reflector 108 is arranged below a lower face 144 of the light guide 104. A light blocking-member 109 is arranged on an upper surface or light emitting face 143 of the light guide 104 in an end region on the side of the light incident end face 141. A light deflecting prism sheet 106 is arranged over the upper surface or light emitting face 143 of the light guide 104 with the light blocking-member 109 interposed therebetween. The above constituent members are set in a casing 113. A lid 114 of light transparency is attached to the casing 113. There is provided on a lower surface of the lid 114 a pressing member 110 by which an end region of an upper surface of the prism sheet 106 is pressed toward the light guide 104.
In such a case, however, there is caused a gap between the light guide 104 and the prism sheet 106 at a region of the light emitting face of the light guide other than the end region thereof on the side of the light incident end face. Accordingly, there is the following problem. That is, as compared with a case of no gap, i.e. a case where the light emitting face of the light guide and the prism sheet are in close contact with each other, there is caused a longer dark shadows or dark parts appearing at outside portions of both ends of a one-dimensional arrangement of a plurality of LED's 102 and at an intermediate portion between the adjacent LED's as shown in FIG. 13. Such dark shadows or dark parts are easily observed within an effective lighting area F, i.e. a lighting area contributing to irradiation of display screen of a liquid crystal display device constituted with a combination of the planar light source device and a liquid crystal display element, of the planar light source device, which corresponds to the display screen of the liquid crystal display device.
Additionally, in a case where the lighting portion of the LED is partly located higher than the upper edge of the light incident end face of the light guide, i.e. the edge thereof on the light emitting face side, there is another problem that luminance becomes lower because the light emitted from the LED is not efficiently introduced into the light guide.
On the other hand, there is proposed a planer light source device having a linear light source such as a cold cathode tube as the primary light source in which an edge of a reflector for the primary light source is extended to cover the end region of the light emitting face of the light guide on the side of the light incident end face, to thereby block the light emitted from the end region of the light emitting face, as described in JP (A)-6-174934 or JP (A)-8-94848. However, these publications do neither suggest the above-mentioned problem that the dark shadows or dark parts are easily observed within the effective lighting area of the planar light source device at the intermediate portion between the adjacent LED's or the like when using the point light source such as LED as the primary light source, nor suggest measure against such a problem.
Furthermore, there is proposed a planer light source device having a linear light source such as a cold cathode tube as the primary light source in which an edge of a reflector for the primary light source or edge of the lid attached to the casing is extended to an end region of the light incident end face of the light guide, to thereby block the light directly incident on an end face of the prism sheet, as described in JP(A)-8-327828. However, these publications also do neither suggest the above-mentioned problem that the dark shadows or dark parts are easily observed within the effective lighting area of the planar light source device at the intermediate portion between the adjacent LED's or the like when using the point light source such as LED as the primary light source, nor suggest measure against such a problem. In addition, in this planar light source device, since the end region of the light incident end face of the light guide is covered with the edge of the reflector or edge of the lid, there is also the above problem that the light emitted from the primary light source is not efficiently introduced into the light guide and therefore the luminance becomes lower.
Furthermore, for the portable electronic devices such as cellular phones, etc., lightening of the devices have been more and more demanded so that it has been proposed to eliminate the above-mentioned casing for setting the constituent members therein. With the constitution proposed in the above JP(A)-6-174934, JP(A)-8-94848 or JP(A)-8-327828, if the casing is eliminated, there is formed a gap between the light guide and the prism sheet also at the end portion of the prism sheet. Accordingly, there is a problem that the light entered the gap between the light guide and the prism sheet is introduced into the prism sheet and emitted therefrom in an oblique direction, to thereby cause the bright line, in addition to the problem that the dark shadows or dark parts are easily observed within the effective lighting area of the planar light source device at the intermediate portion between the adjacent LED's or the like.